Preparation of sols



Patented June 5, 1945 PREPARATION OF SOLS Morris D. Marshall, Arlington,Mass., assignor to Monsanto Chemical Company, St. Louis, Mo., acorporation of Delaware No Drawing. Original application April 30, 1942,

Serial No. 441,212, now Patent No. 2,356,773, dated August 29, 1944.Divided and this application June 28, 1944, Serial No. 542,631

'1 Claim.

This invention relates to the preparation of sols, and particularly toan improved method of preparing relatively stable and substantially saltfree aqua-organosols of unusually high colloidal oxide content.

According .to prior methods it has been possible to form sols having afairly high colloidal oxide content, but these methods have not beenvery satisfactory, particularly .for commercial operations. Silicaa'quasol, for example, has been prepared by reacting analkalisilicat'ewith a mineral acid, removing the inorganic salt from the resultingsolution by dialysis, and then concentrating the remaining aquasol byeva oration. This, however, is a long and drawn out process, primarilybecause salt removal by dialysis is slow. Moreover, dialysis requiresdelicate handling and skilled workmanship, which interferes to a largeextent with successful manufactureon a com-. mercial scale. I It is alsonot possible by this method to obtain sols which are substantially freeof salt and which at the same timehave a silica content higher thanabout 20% by weight.

It is accordingly a primary object of'this invention to provide methodsof making substantially salt free sols which contain an exceptionallyhigh concentration ofa colloidal inorganic oxide (as high as 40% byweight) and are stable over relatively long periods of time.

A- further object of the invention is to provide methods of making solsof the type described which methods are thoroughly adapted for largescale operations. 7

Still further objects and advantages of the invention will appear fromthe following description and appended claim. Before explaining indetail the present invention, however, it is to be ly water, and anupper layer in which the volatile component is predominantly organic.The lower layer contains practically all of the colloidal oxide, and isin the form of an aquaorganosol, which expression will hereinafter beused to distinguish the sols prepared as described herein fromorgano-aquasols, such as the intermediate sol referred to above, whichcontains a preponderant amount of organic solvent. The lower layer maybe separated from the upper layer by drawing off, or by decantaition ofthe upper layer, or by any similar operation.

, In the preparation of the initial aquasol the methods for preparingaquagels'or hydrogels described in the Kistler Patent No. 2,093,454 maybe followed, but gelation is prevented by proper control of pH. Thus,when preparing silica aquasol, for example, by admixing a mineral acidand an alkali silicate solution, it is essential to operate on the acidside, for most purposes at a pH between 1.5 and 4, but preferablybetween 1.5 and 3. This makes possible a long period between the solformation and the precipitation of the gel, for example, as long as 30or 40 days. Where it is desirable, however, the preparation of theaquasols may be carried out at a pH as high as 6, since then also thereis an appreciable period between sol formation and gelation, thuspermitting removal of the inorganic salt formed.

A wide variety of both water-miscible andwa:

ter-immiscible organic solvents may be used according to the invention.Suitable water-misci: ble organic solvents, for example, includeacetone, methyl alcohol, ethyl alcohol, isopropyl alunderstood that theinvention is not limited in its application to the details describedherein,

since the invention is capable of other embodi-v solvent in an amountsufilcient to precipitate the inorganic salt, removing the precipitatedsalt from the resulting organo-aquasol, as for example by filtering,centrifuging, decantation, or the like, and finally addingto theorgano-aquasol solvent, usually about an equal volume, to cause theformation of two layers, i. e., a lower layer in which the volatilecomponent is predominanta sufllcient quantity of water-immiscibleorganic cohol and normal propyl alcohol. Suitable water-immiscibleorganic solvents include higher alcohols, such as the butyl or amylalcohols; alkyl acetates, such as ethyl, butyl or amylacetate; ethers,such as methyl or ethyl ether; and' hydrocarbons, such as lactol spiritsand toluene. A.further understanding of the invention will be obtainedfrom the following examples:

Example I Three hundred and fifty-eight (358) pounds of sodium silicatecomprising 28.7% SiOz, 8.9% Na2O and 62.4% H2O are diluted with 162pounds of water. This mixture is added to 163.5 pounds of 31% sulfuricacid and the mass is agitatedcipitate are separated by filtration. Butylacetate is then added to the resulting acetone-aquasol in an amount byvolume equal to the volume of the sol, which causes a clear colorlessaquaorganosol, containing about 98% of the silica and about 60% of thewater originally present, to separate as a lower layer. off at thebottom, or the upper layer is removed by decantation. The resultingaqua-organosol is substantially pure, i. e., free of salt, and has aconcentration of silica of about 28% by weight.

As a variation of the above, more water may be added to theacetone-aquasol before adding the acetate, in.which case a more dilute$01 is obtained in the lower layer, since approximately the same amountof water is retained by the upper or acetone-acetate layer even thoughrelatively large variations are made in the total water originallypresent.

Example II Eight hundred (800) pounds of sodium silicate containingapproximately 9% NazO and 29% SiOz are mixed at room temperature with350 pounds of water. This mixture is slowly injected at the point ofmaximum agitation into 335 pounds of 31% H2804 which has been cooled toC. and which is being stirred by a high speed impeller. The temperatureduring the mixing is kept below 10 C., and the silicate is added untilthe pH is about 1.8, or until about 90% of the silicate has been added.Three thousand two hundred (3200) pounds of acetone which has beencooled to 0 C. are next added, slowly at first, then more rapidly. Theresulting mixture, which is in the form of an organo-aquasol andcontains precipitated sodium sulfate crystals (Na2SO4'10I-I2O), is thencooled to 0 C. and settled at a temperature between 0 and 5 C.Approximately 85 to 90% of the slightly opalescent sol is then decantedfrom the sodium sulfate crystals with th result that about 99% of thetotal sodium sulfate formed is removed.

Three thousand five hundred (3500) pounds of butyl acetate are added to3600 pounds (about an equal amount by volume) of the above sol, afterbringingit to room temperature, and the mixture is stirred rapidly forabout 2 minutes. A heavy predominantly aqueous layer containing about98% of the total silica and 60% of the total water (about 28% SiOz byweight) separates immediately. Upon removin this aqueous layer bydrawing oil and further extracting it with 2700 pounds of butyl acetatean aqua-organosol of about 40% S102 content is formed.

Example III A silica aquasol containing dissolved sodium sulfate isprepared as described in Example II. Three thousand two hundred (3200)pounds of ethyl alcohol which has been cooled to 0 C. are then added,and the resulting mixture, which is in the form of an organo-aquasol andcontains precipitated sodium sulfate crystals (NAaSOr 101-120) is cooledand settled at a temperature between 0 and 5 C. The sol thus formed isthen separated from the sodium sulfate by decantation. Three thousandfive hundred (3500) pounds of toluene are then added to 3600 pounds ofthe above 501. after bringing it to room temperature, and the contentmay be obtained by further extracting the aqueous layer with anadditional quantity of toluene.

While the examples have been limited, in the interests of brevity, tomethods of preparing sols of silica, it will be apparent that salt freeaqua This is either drawn I organosols of other oxides may be preparedin a similar manner, that is, by forming the proper salt-containingaquasol, precipitating out the inorganic salt formed by adding aquantity of a water-miscible organic solvent, removing the precipitatedsalt, and separating the aqua-organosol from the greater portion of thewater-miscible organic solvent by' adding a water-immiscible organicsolvent. Moreover, other water-miscible organic solvents may besubstituted for the acetone and ethyl alcohol, and otherwater-immiscible solvents may be substituted for the butyl acetate andtoluene described in the examples.

It is also possible by selecting the solvents used in any step, and/or.by varying the proportions of the water-miscible organic solvents andthe aquasol used, to vary the percent by weight of silica or other oxidepresent in the final purified aqua-organosol. In general, it isdesirable to use the water-immiscible solvent in an amount by volumesubstantially equal to the volume of the organo-aquasol being treated,as smaller amounts of the water-immiscible organic solvent, particularlywhen the ratio of the water-immiscible solvent to the organo-aquasol isdecreased by more than about 20%, tend to decrease its efficiency, whilelarger amounts of the water-im- Higher temperatures than this may beused, but I it is usually preferable not to permit the temperature torise above about 20 C. during the preparation of the organo-aquasolbecause of the lower efficiency of the salt precipitation and the dangerof gelation. Moreover, temperatures below 0 C. can be used after theaddition of the water-miscible organic solvent. if'desired, provided thetemprature..,is kept above the freezing range of the mixture.

The relatively stable and substantially salt free aqua-organosols ofhigh oxide content prepared according to this invention may be used forvarious purposes, but they are, as far as now known, best suited formaking relatively dense aerogels of the type described in the KistlerPatent No. 2,093,454, as well as relatively dense liquegels of the like.Dried inorganic xerogels, such as dried silica gel, may be made, forexample, by/dehydrating or drying the sols under atmdspheric pressure inthe usual manner. Aerogey are made by autoclaving the sols directly,that is, without '-first forming a gel. The sols of the presentinmixture is stirred rapidly for about 2 minutes.

A heavy predominantly aqueous layer separates immediately, and can beremoved by drawing oil. An aqua-organoscl of exceptionally high silicavention may also be used to advantage, however, as binders for sandmolds, and are valuable film formers, either alone or in combinationwith other coating materials.

Where reference is made herein to watermiscible and water-immiscibleorganic solvents, it is to be understood that rel mce is made to organicsolvents of the type commonly employed in varnishes, lacquers and likecoating compoportions of petroleum crude oil which are derived frompetroleum by distillation and constitute a mixture of hydrocarbonshaving an evaporation rate similar to or close to that of toluene. Theproduct has a specific gravity of .7313 at 60 F. and the following A. S.T. M. distillation range: I

lcrccnt distilled 'lc-mpcrature F. Initial boiling point 200 50%distilled 22) 90% distilled 240 As previously indicated, the volatilecomponent of the aqua-organoso'ls prepared according to this inventionconsists preponderantly of water. Thus, the sols of the presentinvention usually contain not more than '5 to 25% of organic solventsand therefore differ from the sols prepared according to the methodsdescribed and claimed in Patent No. 2,285,449,,granted to me on June 9,1942, which contain a'predominating amount of organicsolvent'or solventsin the volatile component.

This application is a division of my copending application, Serial No.441,212, filed April 30, 1942,- now Patent No. 2,356,773, granted 'Aug.29, 1944 which is a continuation-in-part of my application' Serial No.272,404, filed May 8, 1939, now issued as Patent No. 2,285,449.

What I claim is:

The method of forming an aqua-organosol' composed of colloidal silica,water and a relatively minor amount of organic solvents which comprisesforming an aquasol containing colloidal silica and a dissolved inorganicsalt, adding a sufiicient quantity of ethyl alcohol to precipitate theinorganic salt, removing the inorganic salt precipitated thereby, addinga sufli- -cient quantity of lactol spirits to cause the separation of anaqua-organosol layer, and separating the aqua-organosol layer whichforms. f

MORRIS D. MARSHALL;

